Ultrasonic welding device

ABSTRACT

A method of welding includes providing first and second joining partners, providing a welding apparatus that includes a sonotrode comprising a structured working surface, arranging the first and second joining partners to contact one another, and forming a welded connection between the first and second joining partners by contacting the first joining partner with the structured working surface and vibrating the sonotrode at an ultrasonic frequency, wherein the structured working surface comprises a plurality of apexes, a plurality of nadirs between immediately adjacent ones of the apexes, and planar sidewalls that extend between the nadirs and the apexes, and wherein for each of the apexes the planar sidewalls on either side of the respective apex extend along first and second planes that intersect one another at an acute angle.

BACKGROUND

Welding is one technique used to form electrical interconnections inelectronics applications. For example, power electronics applicationsmay use welding to form mechanical and electrical interconnections inpower modules. To ensure electrical and mechanical reliability of thewelded joint over the usable lifetime of the device, the welded jointmust have a minimum size and strength. Moreover, an area must bereserved around the footprint of the welded joint to allow the weldedmaterial to expand during the welding process. The collective areaneeded to meet these requirements is dictated by the materials andwelding techniques used. By reducing the collective area needed to forma welded joint, a greater proportion of board area can be dedicated toelectronic components, such as semiconductor dies, passive devices, etc.It is therefore desirable to provide a welding process that reduces thefootprint of a welded joint without sacrificing electrical andmechanical reliability.

SUMMARY

An ultrasonic welding apparatus is disclosed. According to anembodiment, the ultrasonic welding apparatus comprises a sonotrodecomprising a structured working surface that comprises a plurality ofapexes, a plurality of nadirs between immediately adjacent ones of theapexes, and planar sidewalls that extend between the nadirs and theapexes, and for each of the apexes the planar sidewalls on either sideof the respective apex extend along first and second planes thatintersect one another at an acute angle.

Separately or in combination, the structured working surface furthercomprises a plurality of curved surfaces that extend between two of theplanar sidewalls, and each of the nadirs are formed by the curvedsurfaces.

Separately or in combination, the curved surfaces form roundeddepressions that extend between the two of the planar sidewalls.

Separately or in combination, for each of the apexes the planarsidewalls on either side of the apex merge with one another to form anacute point at the apex.

Separately or in combination, a radius of the rounded depressions isequal to or less than 0.5 of a pitch of the apexes on either side of therespective rounded depression.

Separately or in combination, the radius of the rounded depressions isbetween to 0.1 and 0.4 of the pitch.

Separately or in combination, the first and second planes intersect oneanother at an angle of between 50 degrees and 70 degrees.

Separately or in combination, the first and second planes intersect oneanother at an angle of between 55 degrees and 60 degrees.

Separately or in combination, a radius of the rounded depressions isbetween 0.2 and 0.3 of a pitch of the apexes on either side of therounded depressions, and the first and second planes intersect oneanother at an angle of between 55 degrees and 60 degrees.

Separately or in combination, the welding apparatus further comprises atransducer that is mechanically coupled to the sonotrode and isconfigured to cause the sonotrode to vibrate at an ultrasonic frequency.

According to another embodiment, the ultrasonic welding apparatuscomprises a sonotrode comprising a structured working surface thatcomprises a plurality of apexes, a plurality of nadirs betweenimmediately adjacent ones of the apexes, and a plurality of curvedsurfaces that extend between immediately adjacent ones of the apexes,and each of the nadirs are formed by the curved surfaces.

Separately or in combination, a vertical displacement between each ofthe nadirs and the apexes on either side of the respective nadir isgreater than 0.5 of a pitch of the apexes on either side of therespective nadir.

Separately or in combination, a vertical displacement between each ofthe nadirs and the apexes on either side of the respective nadir isgreater than 0.5 of a pitch of the apexes on either side of therespective nadir.

Separately or in combination, the vertical displacement is between 0.55and 0.73 of the pitch.

Separately or in combination, the working surface further comprisesplanar sidewalls that extend from the curved surfaces and intersect withone another to form the apexes, the curved surfaces form roundeddepressions that extend between two of the planar sidewalls, and aradius of the rounded depressions is equal to or less than 0.5 of apitch of the apexes on either side of the respective rounded depression.

A method of welding is disclosed. According to an embodiment, the methodcomprises providing first and second joining partners, providing awelding apparatus that comprises a sonotrode with a structured workingsurface, arranging the first and second joining partners to contact oneanother, and forming a welded connection between the first and secondjoining partners by contacting the first joining partner with thestructured working surface and vibrating the sonotrode at an ultrasonicfrequency, wherein the structured working surface comprises a pluralityof apexes, a plurality of nadirs between immediately adjacent ones ofthe apexes, and planar sidewalls that extend between the nadirs and theapexes, wherein for each of the apexes the planar sidewalls on eitherside of the respective apex extend along first and second planes thatintersect one another at an acute angle, or wherein the structuredworking surface further comprises a plurality of curved surfaces thatextend between immediately adjacent ones of the apexes and each of thenadirs are formed by the curved surfaces.

Separately or in combination, the first joining partner is a powermodule terminal, and the second joining partner a structured metalregion of a power module substrate.

Separately or in combination, at least one of the first and secondjoining partners comprise copper.

Separately or in combination, the structured working surface comprisesthe plurality of curved surfaces, each of the curved surfaces formrounded depressions that extend between the two of the planar sidewalls,and the radius of the rounded depressions is between to 0.1 and 0.4 of apitch of the apexes on either side of the rounded depressions.

Separately or in combination, for each of the apexes the planarsidewalls on either side of the respective apex extend along the firstand second planes that intersect one another at the acute angle, and theacute angle is between 50 degrees and 70 degrees.

Separately or in combination, a vertical displacement between each ofthe nadirs and the apexes on either side of the respective nadir isgreater than 0.5 of a pitch of the apexes on either side of therespective nadir.

BRIEF DESCRIPTION OF THE FIGURES

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Embodiments are depicted in thedrawings and are detailed in the description which follows.

FIG. 1 depicts a welding apparatus and joining partners, according to anembodiment.

FIG. 2 depicts a welded joint that is formed by an ultrasonic weldingtechnique, according to an embodiment.

FIG. 3 , which includes FIGS. 3A, 3B and 3C, depicts a cross-sectionalview of a sonotrode that may be used in an ultrasonic welding technique,according to an embodiment. FIG. 3A depicts a cross-sectional view ofthe sonotrode, FIG. 3B depicts a cross-sectional view of the sonotrodewith geometric references pertaining to the pitch and vertical depth ofthe structured working surface of the sonotrode, and FIG. 3C depicts across-sectional view of the sonotrode with geometric referencespertaining to the angle of the sidewalls and radius of curved surfacesbetween the sidewalls, according to an embodiment.

FIG. 4 depicts a welded joint that is formed by an ultrasonic weldingtechnique, according to an embodiment.

DETAILED DESCRIPTION

Embodiments of an ultrasonic welding apparatus and corresponding methodof welding are described herein. The welding apparatus and method areadvantageously well-suited to reliably form a welded joint with highmechanical strength and a well-controlled footprint are describedherein. To this end, the welding apparatus comprises sonotrode with astructured working surface with apexes and nadirs that are arranged inan advantageous geometry. One advantageous aspect of this geometry is asteep angle of orientation in the planar walls of the structured workingsurface that extend between the apexes and nadirs. Another advantageousaspect of this geometry is curved depressions between the planar wallsof the structured working surface which form the nadirs. These featuresindividually or in combination create a large volume for deformedmaterial to flow in between the protrusions of the structured workingsurface, thereby mitigating the amount of material that is pushedoutside of the periphery of the sonotrode during the ultrasonic weldingprocess. The resultant welded joint has greater electrical andmechanical reliability for a given contact area. This can increase thedensity of welded connections and/or decrease the spacing between thewelded connections and other electronic components such as semiconductordevices.

Referring to FIG. 1 , a welding apparatus 100 comprises an anvil 102, asonotrode 104 and a transducer 106. The sonotrode 104 comprises astructured working surface 108. The structured working surface 108 has apattern of protrusions that form a non-planar surface profile. Thetransducer 106 is mechanically coupled to the sonotrode 104 and isconfigured to cause the sonotrode 104 to vibrate at ultrasonicfrequencies in a process that will be described in further detail below.The anvil 102 is a flat and immovable surface that accommodates theplacement of joining partners thereon.

The welding apparatus 100 can be used to form a welded connectionaccording to the following ultrasonic welding technique. First andsecond joining partners 110, 112 are provided and arranged on the anvil102. Generally speaking, the first and second joining partners 110, 112can comprise any materials that can be fused together through acombination of heat and pressure. Examples of these materials includemetals such as copper, aluminium, nickel, gold, silver, bronze, tin,etc., and alloys thereof, and thermoplastics such as polycarbonates,polyethene, etc. The first and second joining partners 110, 112 arearranged on the anvil 102 with the first joining partner 110 disposedabove the second joining partner 112. The structured working surface 108of the sonotrode 104 is brought into contact with the first joiningpartner 110. The transducer 106 causes the sonotrode 104 to vibrate atan ultrasonic frequency. In this context, an ultrasonic frequency refersto any frequency at or above 10 KHz (kilohertz). Exemplary values forthe ultrasonic frequency are in the range of 10 KHz to 150 KHz. Specificfrequency values for the ultrasonic frequency may be 20 KHz, 30 KHz, or40 KHz, for example. The sonotrode 104 may be vibrated in an up-and-downdirection that is perpendicular to the upper surface 114 of the anvil102 or in a side-to-side direction that is parallel to the upper surface114 of the anvil 102. Generally speaking, the amplitude of thevibrations may be in the range of 0.01 mm (millimeters) to 0.1 mm. Thevibration of the sonotrode 104 brings the first and second joiningpartners 110, 112 into a relative oscillating movement with one another.At this time, the first and second joining partners 110, 112 may bepressed together by a normal force. The normal force may be appliedusing, e.g., pneumatic (air) pressure, a hydraulic system, a spindle, anelectromagnetic actor/linear motor, etc. The oscillation of the firstand second joining partners 110, 112 creates friction at the interfacebetween the first and second joining partners 110, 112, which in turnheats the material in the vicinity of the interface between the firstand second joining partners 110, 112. As a result, the first and secondjoining partners 110, 112 fuse together to create a welded joint.

Referring to FIG. 2 , a welded joint 200 that is created by anultrasonic welding process is depicted, according to an embodiment. Inthis example, the welded joint 200 is between two joining partners thatare each substantially pure copper structures. The welded joint 200 isshown from above the upper joining partner that faces the sonotrodeduring the welding process. This welded joint 200 suffers from thedrawback of uneven distribution of material. In particular, the materialof the joining partners is squeezed to the outer periphery 202 in thedepicted direction 208. Moreover, burrs 206 may form at the outerperiphery 202 of the welded joint 200. This expansion of the weldedjoint 200 may cause unwanted electrical shorts and may place unwantedmechanical stress on an underlying substrate underneath the welded joint200.

Referring to FIG. 3A, a structured working surface 108 of a sonotrode104 is depicted, according to an embodiment. The structured workingsurface 108 comprises a plurality of apexes 116 and a plurality ofnadirs 118 arranged in a regular pattern. The apexes 116 are locationson the structured working surface 108 that form local maximums. Thenadirs 118 are locations on the structured working surface 108 that formlocal minimums. Each of the apexes 116 in the plurality may becoextensive with a single plane such that the sonotrode 104 can bebrought into contact with another planar surface and each of the apexes116 contact this planar surface. Likewise, each of the nadirs 118 may becoextensive with a single plane that is vertically offset from thevertical plane of the apexes 116. The structured working surface 108further comprises planar sidewalls 120 that extend between the nadirs118 and the apexes 116. The planar sidewalls 120 are substantiallyplanar surfaces. This means that the planar sidewalls 120 extend along asingle plane to the extent that this practically achievable withinprocess tolerances of manufacturing techniques for forming the sonotrode104. The structured working surface 108 further comprises a plurality ofcurved surfaces 122 that extend between two of the planar sidewalls 120.Each of the nadirs 118 are formed by the curved surfaces 122. That is,the nadirs 118 correspond to a lowest point on the curved surfaces 122.

The cross-sectional profile of the structured working surface 108 shownin FIGS. 3A-3C can be realized by pattern of elongated protrusions thatextend in a direction that is perpendicular to the cross-sectional planeof the figure. Moreover, as can be appreciated from FIGS. 3 and 4 ,these elongated protrusions can have a have a crisscrossed pattern suchthat the cross-sectional profile of the structured working surface 108shown in FIGS. 3A-3C exists in two different cross-sectional planes thatare transverse (e.g., perpendicular) to one another.

Referring to FIG. 3B, the structured working surface 108 is configuredsuch that the apexes 116 are arranged at a pitch. The pitch refers tothe lateral separation distance D1 between two immediately adjacent onesof the apexes 116. Each of the apexes 116 in the plurality may beregularly spaced apart from one another by this lateral separationdistance D1. Generally speaking, the pitch of the apexes 116 may be anyvalue in the range of 0.05 mm to 5.0 mm. More typically, the pitch ofthe apexes 116 is in the range of 0.1 mm to 1.0 mm. In one particularembodiment, the pitch of the apexes 116 is 0.5 mm.

Additionally, the structured working surface 108 is configured such thatthe apexes 116 and the nadirs 118 have a vertical displacement. Thevertical displacement is a vertical separation distance D2 between thelowest point of the structured working surface 108 (i.e., one of thenadirs 118) and the highest point of the structured working surface 108(i.e., one of the apexes 116). The structured working surface 108 mayhave the same vertical displacement across the pattern of apexes 116 andnadirs 118, meaning that each of the apexes 116 are coextensive with afirst plane and each of the nadirs 118 are coextensive with a secondplane, and the vertical displacement corresponds to a separationdistance between the first and second planes.

According to an embodiment, the vertical displacement between the nadirs118 and the apexes 116 is greater than 0.5 of the pitch of the apexes116. In an embodiment, the vertical displacement may be between 0.51 and1.0 of the pitch. In an embodiment, the vertical displacement is between0.55 and 0.75 of the pitch. In absolute terms, the structured workingsurface 108 can have the following relationships between the verticaldisplacement and the pitch: a vertical displacement of 0.283 mm and apitch of 0.5 mm which corresponds to a ratio of 0.566, a verticaldisplacement of 0.333 mm and a pitch of 0.5 mm which corresponds to aratio of 0.666, or a vertical displacement of 0.364 mm and a pitch of0.5 mm which corresponds to a ratio of 0.728.

Referring to FIG. 3C, the structured working surface 108 is configuredsuch that for each of the apexes 116 the planar sidewalls 120 on eitherside of the respective apex 116 extend along first and second planes124, 126 that intersect one another at an acute angle α. An acute anglerefers to an angle that is less than 90 degrees. Accordingly, in thisembodiment, the protrusion of the structured working surface 108 whichform the apexes 116 comprises outer surfaces (i.e., the planar sidewalls120) that are oriented closer to one another than 90 degrees. Accordingto an embodiment, the first and second planes 124, 126 intersect oneanother at an angle α of between 50 degrees and 70 degrees. In onespecific embodiment, the first and second planes 124, 126 intersect oneanother at an angle α of 55 degrees. In one specific embodiment, thefirst and second planes 124, 126 intersect one another at an angle α of60 degrees.

Additionally, the structured working surface 108 is configured such thatthe curved surfaces 122 form a rounded depression between twoimmediately adjacent ones of the planar sidewalls 120. That is, thecurved surfaces 122 are configured to be equidistant to foci 128,wherein the distance to the foci 128 corresponds to a radius 130.According to an embodiment, the radius 130 of the rounded depressions isequal to or less than 0.5 of the pitch of the apexes 116. According toan embodiment, the radius 130 of the rounded depressions is between 0.1and 0.4 times the pitch. In absolute terms, the structured workingsurface 108 can have the following relations between the radius 130 ofthe rounded depressions and the pitch: a radius 130 of 0.15 mm and apitch of 0.5 mm which corresponds to a ratio of 0.333, or a radius 130of 0.1 mm and a pitch of 0.5 mm which corresponds to a ratio of 0.2.

The geometric features of the structured working surface 108 describedwith reference to FIG. 3 mitigate the non-ideal characteristics of thewelded joint described with reference to FIG. 2 . By way of comparison,a sonotrode may comprise a structured working surface with completelyplanar sidewalls that extend between acute apex and acute nadirs betweeneach of the apex points, wherein the angle of intersection at each ofthe apex points and the nadirs is 90 degrees. In that case, the verticaldisplacement between the apexes and the nadirs is exactly 0.5 times thepitch of the apexes. Such a configuration may cause the problem of thematerial being squeezed out to the periphery of the welded joint 200 asdescribed with reference to FIG. 2 . The structured working surface 108described with reference to FIG. 3 has an increased volume for displacedmaterial to accumulate between the protrusions in comparison to theabove-described arrangement wherein the angle of intersection at each ofthe apex points and the nadirs is 90 degrees. In more detail, byreducing the intersection angle of the first and second planes 124, 126,the width of the protrusions which form the apexes 116 is narrowed, andthus the area between each of the protrusions is increased. Providingcurved depressions in between each of the protrusions may also allow foran increase in the cross-sectional area between each of the apexes 116.Moreover, the curved depressions are also beneficial on the movement ofsqueezed material such that a beneficial impact on the homogeneity ofthe welded joint to be described below with reference to FIG. 4 . may beobserved by incorporating this feature alone.

Referring to FIG. 4 , a welded joint 400 that is created by anultrasonic welding process is depicted, according to an embodiment. Inthis case, the welded joint 400 has been formed using a sonotrode 104which comprises the planar sidewalls 200 extending along first andsecond planes 124, 126 that intersect one another at an angle of 55degrees and which comprises a rounded depression which has a radius 130that is 0.2 times the pitch of the apexes 116. As can be seen, thewelded joint 400 of FIG. 4 has improved characteristics in comparison tothe welded joint 200 of FIG. 2 . In particular, less material issqueezed to the outer periphery of the welded joint 400. The weldedjoint 400 has greater homogeneity and has fewer burrs at its outerperiphery. The likelihood of electrical short and/or the stress placedon the underlying substrate below the welded joint 400 is thereforereduced in comparison to the welded joint 200 of FIG. 2 .

Other variations of the structured working surface 108 than theembodiments specifically discussed with reference to FIG. 3 are possibleto obtain an arrangement wherein the vertical displacement is greaterthan 0.5 of a pitch and the above-discussed beneficial increased volumeto accommodate squeezed material may be realized. A non-exhaustive listof these variations includes the following. Instead of a configurationwherein the planar sidewalls 120 intersect one another to form theapexes 116 as acute points, the protrusions may have a flat or curvedsurface which forms the apexes 116. In that case, a verticaldisplacement of greater than 0.5 of the pitch can be obtained withsufficiently steep angle of inclination for the planar sidewalls 120and/or through appropriately deepened curved surfaces 122 which form thenadirs 118. Instead of a configuration wherein the curved surfaces 122forming the nadirs 118 are perfectly rounded, the curved surfaces 122may have a partial elliptical geometry with two focus 128 points. Inthat case, a vertical displacement of greater than 0.5 of the pitch canbe obtained with sufficiently steep angle of inclination for wherein theplanar sidewalls 120 and/or through appropriately configuring the curvedsurfaces 122. Instead of a configuration wherein the nadirs 122 areformed by curved surfaces, the structured working surface 108 mayfurther comprise trench-like features (e.g., square shaped trenches)disposed between the apexes 116 and adjoining planar sidewalls 120,thereby moving the nadirs 112 further away from the apexes 116 andproviding increased volume in the cross-sectional plane.

Generally speaking, the sonotrode 104 described herein is compatiblewith any ultrasonic welding method, and may be used to form a weldedconnection between any two joining partners. In one particularembodiment, the sonotrode 104 is used to form a welded connectionbetween a structured metal region of a power module substrate and apower module terminal. The power module substrate may be an isolatedmetal substrate (IMS), a direct copper bonding (DCB) substrate, or anactive metal brazed (AMB) substrate, for example. In each case, thepower module substrate comprises a substrate electrically insulatingmaterial, e.g., ceramic, and layer of structured metallization e.g.,copper, aluminum, etc., and alloys thereof disposed on an upper surfaceof the substrate. The power module terminal may be an elongated post ofconductive material, e.g., copper, aluminum, etc., and alloys thereof,which is used to form an externally accessible connection to the devices(e.g., chips, passives, etc.) mounted on the power module substrate. Inthis example, an electrically isolated island in the structuredmetallization region may be welded to the power module terminal by thetechnique described with reference to FIG. 1 , wherein an isolatedregion of the metallization corresponds to the second joining partner112 and a portion of the power module terminal corresponds to the firstjoining partner 110.

Spatially relative terms such as “under,” “below,” “lower,” “over,”“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first,” “second,” and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having,” “containing,” “including,”“comprising” and the like are open-ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a,” “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

1. A method of welding, comprising: providing first and second joiningpartners; providing a welding apparatus that comprises a sonotrode witha structured working surface; arranging the first and second joiningpartners to contact one another; and forming a welded connection betweenthe first and second joining partners by contacting the first joiningpartner with the structured working surface and vibrating the sonotrodeat an ultrasonic frequency; wherein the structured working surfacecomprises a plurality of apexes, a plurality of nadirs betweenimmediately adjacent ones of the apexes, and planar sidewalls thatextend between the nadirs and the apexes, wherein for each of the apexesthe planar sidewalls on either side of the respective apex extend alongfirst and second planes that intersect one another at an acute angle, orwherein the structured working surface further comprises a plurality ofcurved surfaces that extend between immediately adjacent ones of theapexes and each of the nadirs are formed by the curved surfaces.
 2. Themethod of claim 1, wherein the first joining partner is a power moduleterminal, and wherein the second joining partner a structured metalregion of a power module substrate.
 3. The method of claim 1, wherein atleast one of the first and second joining partners comprise copper. 4.The method of claim 1, wherein the structured working surface comprisesthe plurality of curved surfaces, wherein each of the curved surfacesform rounded depressions that extend between the two of the planarsidewalls, and wherein the radius of the rounded depressions is betweento 0.1 and 0.4 of a pitch of the apexes on either side of the roundeddepressions.
 5. The method of claim 1, wherein for each of the apexesthe planar sidewalls on either side of the respective apex extend alongthe first and second planes that intersect one another at the acuteangle, and wherein the acute angle is between 50 degrees and 70 degrees.6. The method of claim 5, wherein a vertical displacement between eachof the nadirs and the apexes on either side of the respective nadir isgreater than 0.5 of a pitch of the apexes on either side of therespective nadir.
 7. The method of claim 1, wherein a verticaldisplacement between each of the nadirs and the apexes on either side ofthe respective nadir is the same throughout the plurality of apexes andthe plurality of nadirs.
 8. The method of claim 1, wherein the first andsecond planes intersect one another at an angle of between 50 degreesand 70 degrees.
 9. The method of claim 8, wherein the first and secondplanes intersect one another at an angle of between 55 degrees and 60degrees.
 10. The method of claim 1, wherein the welding apparatusfurther comprises a transducer that is mechanically coupled to thesonotrode and is configured to cause the sonotrode to vibrate at anultrasonic frequency.